Field of the Invention
The present invention relates to a thin film transistor substrate, a method for manufacturing the same, and a liquid crystal display.
Description of the Background Art
Thin film transistor active matrix substrates (thin film transistor substrates (TFT substrates)) including thin film transistors (TFTs) as switching elements find use in electro-optical apparatuses such as displays (liquid crystal displays) including liquid crystals. The semiconductor devices such as the TFTs have features of being power-thrifty and thin, thus being used in place of cathode ray tubes (CRTs) and finding increasing applications to flat panel displays.
The electro-optical elements for use in liquid crystal displays (hereinafter also referred to as “LCDs”) include passive matrix LCDs and TFT LCDs including TFTs as switching elements. In particular, the TFT LCDs are superior in portability and display quality to the CRTs and the passive matrix LCDs, thus finding widespread practical applications to display products such as notebook computers and TVs.
In general, the TFT LCD includes a liquid crystal display panel in which a liquid crystal layer is sandwiched between a TFT substrate and a counter substrate. The TFT substrate includes an array of a plurality of TFTs and the counter substrate includes, for example, a color filter. The liquid crystal display panel includes polarizing plates located on the front surface side and the back surface side and a backlight located on one of these sides. This structure provides an excellent color display.
The LCDs employing the in-plane switching (IPS) (registered trademark) mode have the advantage in wide viewing angle and thus are widely used in, for example, displays. The IPS mode is the transverse electric field liquid crystal driving mode yielding an improvement in the viewing angle of the conventional TFT LCDs. Unfortunately, such LCDs have a low aperture ratio and a low transmittance in the pixel display portion, and thus, can hardly have bright display properties. It is considered that the following factor plays a major role in interfering with the bright display properties. The electric field for driving liquid crystals fails to work properly above the comb-teeth shaped pixel electrode included in the IPS LCD, so that a part of the liquid crystals located above the pixel electrode is not driven. To solve this problem, the LCD disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-56474 has been proposed which employs the fringe field switching (FFS) mode.
For the switching element of the TFT substrate included in the conventional liquid crystal display, amorphous silicon (Si) has been used as the semiconductor film being the active layer (channel layer). In recent years, the TFTs including an active layer made of an oxide semiconductor have been actively developed. Such oxide semiconductor has a higher mobility than that of the conventionally-used amorphous silicon. The oxide semiconductor is mainly the material based on zinc oxide (ZnO) or the material based on amorphous InGaZnO obtained by adding gallium oxide (Ga2O3) and indium oxide (In2O3) to zinc oxide. This technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-56474, Japanese Patent Application Laid-Open No. 2005-77822, and Kenji Nomura, et al., “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, no. 432 (2004): 488-492.
Similarly to the oxide conductors being the transparent conductors such as amorphous ITO (indium oxide (In2O3)+tin oxide (SnO2)) and amorphous InZnO (indium oxide (In2O3)+zinc oxide (ZnO)), the above-mentioned oxide semiconductor can be etched with a weak acid solution containing oxalic acid or carboxylic acid, thus having the advantage of being easily patterned.
However, in some cases, the oxide semiconductor is damaged by the acid solution used to etch a general metal film which is to be formed into a source electrode and a drain electrode of the TFT, resulting in characteristics degradation. The oxide semiconductors made of certain kinds of material may dissolve in the above-mentioned acid solution. Thus, as described in Japanese Patent Application Laid-Open No. 2007-281409, for the manufacturing of the TFT (generally referred to as back channel etch (BCE) TFT) including the source electrode and the drain electrode as an upper layer on the oxide semiconductor film, the oxide semiconductor film being the channel layer is damaged by the acid solution in some cases during the processing of the source electrode and the drain electrode, resulting in the degradation of the TFT characteristics. In other cases, the oxide semiconductor film being the channel layer is damaged due to the oxidation-reduction reaction in the interface during the deposition of the metal film which is to be formed into the source electrode and the drain electrode, causing the degradation of the TFT characteristics.
These problems can be solved by applying the TFT structure that includes, as an upper layer on the semiconductor layer, a protective layer to protect the semiconductor layer as described in Japanese Patent Application Laid-Open No. 62-235784 (1987) and International Publication No. WO 2011/077607. Such TFT structure can prevent or reduce damage to the oxide semiconductor film during the etching of the metal film which is to be formed into the source electrode and the drain electrode or loss of the oxide semicondcutor resulting from the etching. This structure is generally referred to as “etching stopper (ES) TFT” or “etch stopper (ES) TFT” as a reflection of the above-mentioned function.
The application of the etch stopper TFT structure to the TFTs of the general FFS LCD disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-56474 requires an additional photolithography process dedicated to the formation of the protective layer on the oxide semiconductor layer. Unfortunately, the additional photolithography process required in the manufacturing process reduces the production capacity and increases the manufacturing cost. The increased manufacturing cost resulting from the application of the etch stopper TFT structure is the problem associated not only with the FFS LCDs but also with other LCDs in which the TFTs including the active layer being the oxide semiconductor serve as switching elements.
If the general metal film which is to be formed into the source electrode and the drain electrode is deposited directly onto the oxide semiconductor film by sputtering or chemical vapor deposition in the state where the channel layer (region) of the semiconductor film is protected by the etch stopper layer, the interface reaction (the oxidation-reduction reaction) would occur in the source region and the drain region of the semiconductor film that are electrically connected with the source electrode and the drain electrode. If this is the case, the oxidation-reduction reaction in the oxide semiconductor film would have a spread effect across the semiconductor layer including the channel region, resulting in the degradation of the TFT characteristics.